DE3650609T2 - Ultra high molecular weight polyethylene wire and process for making same - Google Patents

Description

The present invention relates to a whitened wire made of ultra-high molecular weight polyethylene which is plastically deformable and to a method for producing the wire.

Ultra-high molecular weight polyethylene is increasingly used as a so-called engineering plastic because of its excellent impact resistance, abrasion resistance, chemical resistance, tensile strength and other properties compared with conventional polyethylene. However, ultra-high molecular weight polyethylene has extremely high melt viscosity and low flowability compared with conventional polyethylene, and it is therefore very difficult to mold such a plastic by a conventional extrusion molding method and a conventional injection molding method. As a result, ultra-high molecular weight polyethylene is generally molded by compression molding; except for rods and the like, which are partially extruded at a very low extrusion speed.

On the other hand, for example, in Japanese Examined Patent Publication (Kokoku) No. 37-9765, as a method for drawing the monofilament of a high density polyethylene with a high degree of drawing, it has been proposed that an additive (for example, o-chlorobenzene, xylene) having a boiling point higher than the melting point of the polyethylene is incorporated into the polyethylene in an amount of 20 wt% to 150 wt% based on the weight of the polyethylene and that the resulting dispersion is made into a primary fibrous material is formed, followed by hot stretching at a stretching ratio of 3 to 15 while maintaining the amount of additive corresponding to 5 wt% to 25 wt% in the spun filaments. It has also been proposed, for example, in Japanese Unexamined Patent Publication No. 56-15408 that a solution of linear polyethylene having a molecular weight of 400,000 or more is spun, followed by stretching the same at such a temperature that the modulus of at least 20 GPa can be achieved. However, although these methods can give stretched filaments having high elasticity and high strength, raw filaments having a diameter of more than 1 mm are very difficult to obtain because a solution having an extremely low viscosity obtained by diluting ultra-high molecular weight polyethylene or the like with a large amount of a solvent is extruded and processed. As a result, the drawn thread generally has a thickness of 10 denier or less (i.e., diameter: 0.038 mm or less) and a thickness of about 50 denier (0.190 mm) or less at most. Accordingly, since a large amount of these drawn fine threads must be entangled to form thick ropes and nets having a high strength or cords for cord cutting machines (i.e., lawn mowers), problems arise in that the operations of the process are cumbersome, the rigidity of the resulting ropes and nets is unpreferably high, and the flexibility is insufficient. In addition, the resulting ropes and nets are usually very hairy.

In addition, for example, in Japanese Unexamined Patent Publication (Kokai) No. 52-74682, it has been proposed that polymers having an average molecular weight of 150,000 or more are processed under the conditions of a temperature of about 75°C to about 140°C and at a strain rate of about 18 or more to form elongated and oriented polymers. However, this document describes that the diameter of fibers or the thickness of films or tapes before stretching is preferably about 1 mm or less. Furthermore, the examples given herein merely disclose that polyethylene having a molecular weight of about 300,000 is spun through circular orifices having a diameter of 1 mm to form thin filaments having a diameter of 0.7 mm or less and that in the case of polyethylene having a molecular weight of 800,000, the filaments are merely prepared by stretching dumbbell-shaped samples having a gauge of 1 cm x 0.2 cm derived from the pressed layer having a thickness of 0.5 mm. Thus, it has not been demonstrated in the prior art that filaments having a diameter of more than 1 mm can be industrially produced from an ultra-high molecular weight polyethylene having an extremely high molecular weight.

Furthermore, in the prior art, there has been a desire to replace the iron wires that are commonly used in orchards, for example, for securing the loculi of fruit or for vine trellises, with plastic wires because iron wires rust easily and cannot withstand the effects of long-term use. However, conventional commercially available plastic wires composed of nylon or polypropylene have a certain elasticity and therefore, unlike iron wires, cannot be plastically deformed. For example, plastic wires cannot be firmly connected merely by twisting.

The present invention provides a whitened wire made of ultra-high molecular weight polyethylene which is plastically deformable.

The main EP application No. 86 300 614.4 provides an extruded stretched filament of ultra-high molecular weight polyethylene having an intrinsic viscosity [η] of at least 3.5 dl/g, a tear strength of at least 10 kg/mm2, a degree of stretching of at least 3, a degree of C-axis orientation of at least 0.9 and an outer diameter of 0.1 to 10 mm (and an essentially arbitrary length).

It further provides a process for producing an extruded filament from ultra-high molecular weight polyethylene, said process comprising the following steps:

Melting the ultra-high molecular weight polyethylene in a screw extruder;

Extruding the molten ultra-high molecular weight polyethylene from a die having an L/D of at least 10; and

Removing the extrudate at a draw ratio of at least 1, while gradually cooling the extruded strands.

It further provides a device for producing an extruded filament of ultra-high molecular weight polyethylene, said device having in succession from one extruder side:

An extruder provided with a grooved barrel and a screw with a compression ratio of 1 to 2.5;

A nozzle connected to the screw tip, the nozzle having an L/D of at least 10 and a ratio S₁/S₂ of a cross-sectional area S₁ of the inlet region of the nozzle to a cross-sectional area S₂ of the outlet region of the nozzle of 1.0 to 5.0;

A staged cooling cylinder provided with an air ring or a sizing nozzle at its inlet area for the extruded strand; and

A acceptance unit.

In accordance with the present invention, a whitened wire made of ultra-high molecular weight polyethylene having an intrinsic viscosity [η] (determined at 135ºC in a decalin solvent) of at least 4 dl/g, a tear strength of at least 0.2 GPa (20 kg/mm²) and a fold recovery angle θ of 20º or less when folded at 90º and 180º.

This whitened wire can be manufactured by a process which includes the following steps:

Melting the ultra-high molecular weight polyethylene in a screw extruder;

Extruding the molten ultra-high molecular weight polyethylene from a die having an L/D of at least 10;

Removing the extrudate with a stretch ratio of at least 1 while gradually cooling the extruded strand; and

Stretching the strands in one step or in several steps, for example with a stretching degree of 8 to 30.

For a better understanding of the present invention, the following description in conjunction with the accompanying drawings, in which:

Fig. 1 shows schematically a section through a typical embodiment of an apparatus for producing a stretched filament from ultra-high molecular weight polyethylene;

Fig. 2(a) and (b) schematically show the states of the plastic wire 21 according to the present invention when the plastic wire 21 is folded; and

Fig. 3(a) and (b) schematically show the states of the plastic wire 22 according to the present invention.

The ultra-high molecular weight polyethylenes useful in the present invention are crystalline ethylene homopolymers or crystalline ethylene-alpha-olefin copolymers having a major proportion of ethylene (for example, an ethylene content of about 95 to about 100 mole %), having an intrinsic viscosity [η], determined at 135°C in a decalin solvent, of 4 dl/g or more, more preferably 8 to 25 dl/g, and a melt flow rate (MFR) of 0.01 g/10 min or less, determined according to the ASTM D-1238 (F) method. Examples of alpha-olefin comonomers are those with 3 to 20 carbon atoms, for example propylene, 1-butene, 1-hexene, 1-octene and 4-methyl-1-pentene.

The stretched filaments of ultra-high molecular weight polyethylene can be obtained by stretching raw filaments thereof prepared as explained above. The resulting stretched filaments have an intrinsic viscosity [η'] determined as above of 4 dl/g or more, preferably 8 to 30 dl/g, and a tensile strength (TS) of at least 0.2 GPa (20 kg/mm²) or more, preferably 0.5 GPa (50 kg/mm²) or more; they may have a degree of stretching of at least 3, preferably 6 or more, a degree of C-axis orientation of at least 0.9, preferably 0.95 or more, and an outer diameter of 0.1 to 10 mm, preferably 0.2 to 3 mm. The length of the stretched filaments is substantially arbitrary.

The raw filaments of the stretched filaments having the extremely high molecular weight can be produced by melting the above-mentioned ultra-high molecular weight polyethylene in a screw extruder, preferably a screw extruder provided with a grooved cylinder, and then melt-extruding the molten polyethylene from a die having an L/D of at least 10, preferably 15 or more, more preferably 30 to 60. After extrusion, the extrudate is taken out at a stretch ratio of at least 1, preferably 1.2 to 30, while the extruded strands are gradually cooled, preferably by passing them through a cooling cylinder. having a length of, for example, 0.3 to 5 m and by blowing with air at 15°C to 80°C. Thus, the desired raw filaments of ultra-high molecular weight polyethylene having an outer diameter of generally 0.2 to 20 mm, preferably 2 to 10 mm, and having excellent stretchability can be obtained. The term "stretch ratio" used herein means a ratio of the take-off speed of the take-off unit to the extrusion speed of the extruded polyethylene at the outlet of the die.

When a die having an L/D of at least 10 is used, the molten polyethylene has a low melt flowability and is not completely uniformly melted before it is extruded from the die because the molten ultra-high molecular weight polyethylene is a rubber-like mass unlike the molten product of conventional polyethylene, and therefore only raw filaments having a low stretchability can be obtained. As a result, the stretched filaments having a sufficient strength even after stretching cannot be obtained. On the other hand, although there is no critical limitation on the upper limit of the L/D ratio, from a practical standpoint, the preferable upper limit of the L/D ratio is 100. The shape of the die is generally circular or cylindrical. There is a correlation between the productivity and the L/D ratio of the die. As a result, the larger the L/D ratio, the higher the increase in production rate (i.e., the decrease rate).

When the molten strands extruded from the die are cooled, for example, by means of a water circulation water bath, raw filaments having a low stretchability are obtained because bubbles are generated with a vacuum in the cores of the strands and because the crystallinity becomes low. In addition, in order to obtain stretched filaments having an excellent tear strength from the raw filaments obtained by a stretch ratio of less than 1, were, the degree of stretching must be at least 4 and therefore loses effectiveness.

There are no critical limitations on the extrusion temperature of the ultra-high molecular weight polyethylene, as long as the temperature is higher than the melting point of the polyethylene but lower than the decomposition temperature thereof. The preferred extrusion temperature conditions of the above-mentioned ultra-high molecular weight polyethylene are an extruder temperature of 180°C to 350°C, a temperature between the inlet and the intermediate portion of the tube die of 180°C to 300°C, and temperatures between the intermediate portion and the outlet of the tube die of 135°C to 160°C. Thus, the desired raw filaments of ultra-high molecular weight polyethylene having an outer diameter of 0.2 to 20 mm, preferably 2 to 10 mm, particularly when the wires are produced therefrom, can be obtained.

When the extruded raw filaments are made of the ultra-high molecular weight polyethylene having an outer diameter of 0.2 to 20 mm, either dry stretching or wet stretching can be carried out. However, the use of wet stretching is preferred from the viewpoint of heat transfer efficiency and temperature control accuracy. In addition, the use of multi-step stretching is preferable when compared with single-step stretching because the degree of stretching can be increased and because stretched filaments having a high strength can be obtained.

A typical embodiment of the apparatus suitable for use in producing extruded raw filaments of the above-mentioned ultra-high molecular weight polyethylene will be explained in detail with reference to the accompanying drawing.

As illustrated in Fig. 1, the apparatus for producing raw filaments from the ultra-high molecular weight polyethylene comprises in sequence from the extruder side: an extruder 1 provided with a grooved cylinder 2 and a screw 3 having a compression ratio of 1 to 2.5, preferably 1.3 to 1.8, a cylindrical nozzle 6 connected to the tip of the screw 3, a cooling cylinder 9 and a take-off unit 10. The nozzle 6 has an L/D ratio of at least 10, preferably 15 or more, more preferably 30 to 60 and a ratio S₁/S₂ of a cross-sectional zone S₁ of the inlet region 4 of the nozzle 6 to a cross-sectional zone S₂ of the outlet region 5 of the nozzle 6 of 1 to 5, preferably 1.5 to 3.0. The cooling cylinder 9 for gradually cooling the extruded strands is provided with an air ring or a calibration nozzle (not shown) at the inlet region 8 of the strands.

The grooved cylinder 2 is provided with groove portions 11 to ensure a steady supply of the powder particles of the ultra-high molecular weight polyethylene to a compression zone 12. As mentioned above, the compression ratio of the screw 3 should be within the range of 1 to 2.5. If the compression ratio of the screw 3 is less than 1.0, the contact stress of the polyethylene against the cylinder wall is small, so that the extrusion amount becomes unstable and the resulting extruded filaments will have a poor appearance due to swinging or insufficient venting. In contrast, when the compression ratio of the screw is more than 2.5, the polyethylene temperature is abnormally increased due to clogging and the generation of frictional heat in the compression zone, so that the friction coefficient and the abrasion resistance of the extruded raw filaments are impaired due to a decrease in the molecular weight of the polyethylene caused by thermal decomposition.

The term "compression ratio" of the screw 3 used herein is defined by a ratio of the groove depth at the feed portion of the screw 3 to the groove depth at the screw tip.

The L/D ratio of the extruder 1 is generally 7 to 32, preferably 20 to 26. If the L/D ratio of the extruder 1 is less than 7, it is likely that ultra-high molecular weight polyethylene powder to the cylindrical nozzle 4 due to incomplete melting.

On the other hand, if the L/D ratio of the cylindrical nozzle 6 is less than 10, raw filaments having a low stretchability are formed because the molten product having the extremely high molecular weight is not completely melted before the molten product is extruded from the nozzle. The L/D ratio of the cylindrical nozzle 6 is defined by a ratio of the length L between the inlet portion 4 of the cylindrical nozzle 6 and the outlet portion 5 of the nozzle 6 to the inner diameter D of the nozzle outlet 5. Further, as mentioned above, the ratio S₁/S₂ should be within the range of 1 to 5. If the ratio S₁/S₂ is less than 1, the melting of the molten ultra-high molecular weight polyethylene is incomplete due to insufficient internal pressure and therefore the appearance (or shape) of the extruded strand is deteriorated and the desired raw filaments having excellent stretchability cannot be obtained. In contrast, when the ratio S₁/S₂ is more than 5, a so-called shear dropout current is likely to be generated in the molten product and therefore undesirable extrusion must be carried out at a low speed.

There are no critical limitations on the length of the gradually cooling cylinder 9, as long as its length is sufficient to cool the molten strands extruded from the cylindrical nozzle without quenching. For example, the length of the cooling cylinder 9 is generally 0.3 to 5 m, preferably 1 to 3 m. If the length of the cooling cylinder 9 is less than 0.3 m, the molten strands cannot be cooled gradually accurately when the take-off speed is high. The air ring provided at the inlet portion of the molten strand cooling cylinder 9 is designed to gradually cool the molten strands by supplying warm air having a temperature in the range of 15°C to 80°C, preferably 30°C to 50°C. In addition, a calibration nozzle (not shown) is optionally provided at the inlet side of the cooling cylinder 9 to control the size of the molten strands and to gradually cool these strands.

Preferably, the extruded strands having an intrinsic viscosity [η] (determined as explained above) of 4 dl/g or more, preferably 8 to 30 dl/g, and having a tensile strength of 0.2 GPa (20 kg/mm2) or more, preferably 0.5 GPa (50 kg/mm2) or more, are stretched at a draw ratio of 8 to 30 to form whitened plastic wires. The resulting ultra-high molecular weight polyethylene plastic wires are oriented and crystallized by stretching in the stretching direction. As a result, the wires are divided into fine fibers to form whitened wires, and accordingly, when the wires are bent or twisted, deformation is caused by sliding between the fibers based on the stress. Therefore, even after the stress is removed, such deformation cannot be reversed and the wires cannot be returned to their original linear shape due to their elasticity.

The folding recovery angles θ when the present wire is folded by 90° and 180° are 20° or less, preferably 10° or less. The term "folding recovery angle" used herein means the recovery angle θ after the wires are allowed to remain for 10 minutes under the folding conditions shown in Figures 2 and 3.

If the intrinsic viscosity [η] is less than 4 dl/g, then the average molecular chain lengths are small and therefore the tensile strength cannot be increased even if the stretch ratio is increased. On the other hand, if the tensile strength is less than 20 kg/mm2 or less, the strength is likely to be insufficient to allow the plastic wires to be used instead of iron wires.

The outer diameter of the plastic wires according to the present invention is generally about 0.4 to 3.5 mm, preferably about 0.5 to 2.5 mm. The plastic wires according to the present invention can be used either as a single wire or as thicker wires obtained by twisting a plurality of single wires. If the outer diameter is less than 0.4 mm, the productivity of the raw filaments is low from an industrial standpoint. On the other hand, if the thickness of the wire is more than 3.5 mm, the initial cost of the drawing device is unpreferably large and further no practical application of the wire can be expected.

The stretching temperature is preferably a temperature of the melting point of the ultra-high molecular weight polyethylene plus 15°C or less, preferably 80°C to 150°C, more preferably 100°C to 150°C. Although stretching can be effected even at a temperature higher than the melting point of the ultra-high molecular weight polyethylene plus 15°C, the desired whitened wires having a high strength cannot always be obtained. In contrast, if the stretching temperature is too low, the desired high strength cannot always be obtained although the whitened wires can be obtained.

Although the stretching can be carried out in a single step, multi-step stretching is preferably used because the stretching degree can be increased and because the desired stretched filaments with a high strength can be obtained.

The typical multi-stage stretching conditions are as follows. That is, the raw filaments or strands are stretched to a stretch degree of 4 to 8 at 80°C to 135°C in the first stage, and the stretched filaments are then stretched to a stretch degree of 2 to 5 at 140°C to 150°C in the second stage. Thus, the raw filaments are stretched to a total stretch degree of 8 to 30, preferably a total degree of stretching of 10 to 30. The plastic wires thus obtained are oriented in the direction of stretching and crystallized and whitened.

As explained above, the stretched filaments of ultra-high molecular weight polyethylene, although much thicker than conventional melt-spun filaments or the like having the extremely high molecular weight with an outer diameter of 0.1 to 10 mm, have extremely high tear strength when compared with conventional stretched filaments of ultra-high molecular weight polyethylene, while retaining the inherent characteristics of ultra-high molecular weight polyethylene, for example, abrasion resistance, self-lubricating properties and impact resistance. Accordingly, the present stretched filaments can be advantageously used as, for example, thick ropes and nets for mooring ships or for load-bearing devices or as cords for cord-cutting devices (i.e., lawn mowers), for which the use of conventional ultra-high molecular weight polyethylene filaments is limited.

Furthermore, the above-mentioned manufacturing method and the above-mentioned manufacturing apparatus can stably produce raw filaments of the ultra-high molecular weight polyethylene having excellent stretchability, which filaments are suitable for producing stretched filaments having the characteristics explained above, and it has been difficult to produce such stretched filaments heretofore.

Furthermore, as mentioned above, because the plastic wires made according to the present invention are plastically deformable, these plastic wires can be twisted for fastening in the same manner as iron wires. Accordingly, conventional iron wires can be replaced by the plastic wires according to the present invention in any application in which no substantial heat is applied. For example, the plastic wires according to the present invention can be used for fastening the loculi of fruit and used for forming vine trellises and orchard divisions. In addition, since the plastic wires according to the present invention do not rust unlike iron wires, the present plastic wires have excellent maintainability and can be used repeatedly without breaking the wires due to their plastic deformability.

EXAMPLES

The present invention will now be further illustrated by the following examples, but is by no means limited thereto.

Example 1 and comparative example 1

Stretchable raw filaments (or strands) of ultra-high molecular weight polyethylene were prepared using an apparatus for preparing such stretchable raw filaments (or strands).

The specifications of the device used are as follows:

Screw: outer diameter = 20 mm

Effective screw length (L/D) = 22

Compression ratio = 1.8

Nozzle: Length = 240 mm

Inner diameter at the nozzle outlet = 6 mm

L/D of nozzle = 40

S₁/S₂ = 1.8

Powdered ultra-high molecular weight polyethylene, Hizex Million 240M, available from Mitsui Petrochemical Industries, Ltd. (intrinsic viscosity [η] (in decalin solvent at 135ºC) 16.5 dl/g, melt flow rate (MFR) less than 0.01 g/10 min, melting point 136ºC, bulk density 0.45 g/cm³) was extruded from the above-mentioned extruder under the following conditions extruded to produce stretchable raw filaments or strands of ultra-high molecular weight polyethylene with an outer diameter of 4.0 mm.

Setting the temperature

Extruder = 320ºC

Nozzle base (D�1) = 200ºC

Nozzle tip (D₂) = 140ºC

Screw speed = 50 rpm

Removal speed = 1.5 m/min.

The resulting strands were stretched in a stretching bath containing triethylene glycol at a stretch ratio of 18 (i.e., first step: 6 times at 133°C, second step: 3 times at 144°C) to prepare the stretched filaments.

In Comparative Example 1, conventional polyethylene (intrinsic viscosity [η] (in decalin solvent at 135°C) 2.6 dl/g, melt flow rate 0.11 g/10 min, density 0.956 g/cm3, melting point 131°C) was extruded and stretched in the same manner as in Example 1 except that the extruder temperature was changed to 220°C. Thus, the stretched filaments were prepared.

The physical properties of the resulting stretched filaments were determined according to the following methods. The results were as shown in Table 1.

Density: ASTM D 1505

Diameter: Micrometer

Tensile test: The tensile strength, elastic modulus and elongation (%) were measured at 23ºC under the conditions of a tension rate of 200 mm/min and a chuck distance of 200 mm using an Instron type universal testing machine CATY-1001ZS manufactured by Yonekura Co., Ltd.

Degree of orientation: The degree of orientation was calculated from the following equation by determining the (110) surface reflection using X-ray diffraction apparatuses RU-200A and RU-200PL.

Degree of orientation = 180-αº/180,

where αº is the half-width of (110) surface reflection.

Example 2 and comparative example 2

Stretchable ultra-high molecular weight polyethylene strands having an outer diameter of 8.0 mm were prepared in the same manner as in Example 1 except that a die having a length of 600 mm, an inner diameter at the die outlet of 12 mm, an L/D ratio of 50 and an S1/S2 ratio of 1.9 was used, and the screw revolution number and the take-off speed were changed to 50 rpm and 0.8 m/min, respectively. The strands thus obtained were stretched at a stretch ratio of 8 (i.e., first stage: 4 times at 134°C, second stage: 2 times at 145°C).

In Comparative Example 2, stretched filaments were produced in the same manner as in Example 2 except that the conventional polyethylene used in Example 1 was used instead of the ultra-high molecular weight polyethylene.

The physical properties of the resulting drawn filaments were determined in the same manner as in Example 1. The results are shown in Table 1.

Comparison example 3

An attempt was made to produce stretchable strands from the ultra-high molecular weight polyethylene used in Example 1 in the same manner as in Example 1 except that a die having a length of 50 mm, an inner diameter at the die outlet of 6 mm, an L/D ratio of 8.3 and an S₁/S₂ ratio of 1.1 was used, the set temperatures of the extruder, D₁ and D₂ were changed to 350°C, 230°C and 140°C, respectively, and the screw revolution number and the take-off speed were changed to 25 rpm and 0.7 m/min, respectively. However, the polyethylene was cut off intermittently and continuous stretchable strands could not be produced. Table 1

*1: Stretching impossible due to filament cutting Table 2

*: Routes impossible

Example 3 and comparative example 4

The stretchable strands produced in Example 1 were stretched in a stretching bath containing triethylene glycol at the following stretching levels:

1) 6 times (that is, first stage - stretching at 133ºC)

2) 8 times (that is, first stage - stretching at 133ºC)

3) 10 times (that is, first stage - stretching at 133ºC)

4) 12 times (i.e. first stage: 6 times at 133ºC, second stage: 2 times at 144ºC)

5) 18 times (i.e. first stage: 6 times at 133ºC, second stage: 3 times at 144ºC)

6) 24 times (i.e. first stage: 6 times at 133ºC, second stage: 4 times at 144ºC).

In Comparative Example 4, a conventional polyethylene having an intrinsic viscosity [η] of 2.6 dl/g, a melt flow rate (MFR) of 0.11 g/10 min, a melting point of 131°C and a bulk density of 0.956 g/cm3 was used to produce drawn filaments under the same conditions as in Example 3 except that the extruder temperature was changed to 220°C.

The resulting stretched filaments were evaluated as follows:

Density: ASTM D 1505 (no tempering)

Diameter dimension: Same as in Example 1

Tensile test: Same as in Example 1

Folding return angle:

(1) Recovery angle after 10 minutes from a fold of 180º (see Fig. 2)

(2) Recovery angle after 10 minutes from a 90º fold (see Fig. 3)

The results are shown in Table 2.

As is clear from the results shown in Table 2, when the degree of stretching is 8 or more, more specifically 10 or more, the desired plastic wires having a small recovery angle when they are folded and having excellent tear strength can be obtained. In contrast, when conventional polyethylene is used, the desired plastic wires having a small recovery angle when they are folded cannot be obtained.

Claims (5)

1. Whitened wire made of ultra-high molecular weight polyethylene with an intrinsic viscosity [η] (determined at 135ºC in decalin solvent) of at least 4 dl/g, a tensile strength of at least 0.2 GPa (20 kg/mm²), and a folding recovery angle θ of 20º or less when folded at 90º and 180º. 2. A whitened wire according to claim 1, wherein the intrinsic viscosity number [η] is 8 to 30 dl/g, the tear strength is 0.5 GPa (50 kg/mm²) or more, and the folding recovery angle θ is 10° or less when folded at 90° and 180°. 3. A method for producing a whitened wire according to claim 1, comprising the following steps: Melting the ultra-high molecular weight polyethylene in a screw extruder, Extruding the molten ultra-high molecular weight polyethylene from a die having L/D of at least 10; Removing the extrudate with a stretch ratio of at least 1 while gradually cooling the extruded strand and Stretching the removed strand in one or more stages. 4. The method according to claim 3, wherein the removed strand is stretched with a degree of stretching of 8 to 30. 5. A method according to claim 4, wherein the removed strand is first stretched at 80 to 135°C with a stretch ratio of 4 to 8 and then stretched again at 140 to 150°C with a stretch ratio of 2 to 5 to form the whitened wire with a total stretch ratio of 8 to 30.